The present invention relates to an ion guide, an ion mobility spectrometer or separator, a mass spectrometer, a method of guiding ions, a method of separating ions according to their ion mobility and a method of mass spectrometry.
The analytical utility of providing a forward impetus for ions confined by RF fields in the presence of gaseous media has long been understood. Particular applications include the creation of collision cells for tandem mass spectrometers wherein fast transit times are desirable e.g. when performing MRM, parent ion scanning or neutral loss experiments on triple quadrupole instruments. Such devices may also be used to separate ions according to their ion mobility and these are finding wider use in hybrid ion mobility-mass spectrometer instruments. Typical pressure ranges of operation are in the region of 0.001 to 10 mbar.
Various ion guides are known including an ion tunnel ion guide comprising a plurality of ring electrodes. Opposite phases of an RF voltage are applied to adjacent electrodes so that ions are confined by a pseudo-potential well within the ion tunnel ion guide. A DC travelling wave may be applied to the ring electrodes in order to urge ions along the length of the ion guide. Ions are transmitted and guided along the length of the ion guide passing through the apertures in the ring electrodes. It is also known to provide an ion mobility spectrometer comprising an ion tunnel ion guide maintained at a relatively high pressure. As ions are transmitted through the ion tunnel ion guide, the ions become separated temporally according to their ion mobility.
U.S. Pat. No. 6,914,241 (Giles) describes how ions may be separated according to their ion mobility by progressively applying transient DC voltages along the length of an RF ion guide or ion mobility separator comprising a plurality of electrodes. The ion mobility separator may comprise an AC or RF ion guide such as a multipole rod set or a stacked ring set. The ion guide is segmented in the axial direction so that independent transient DC potentials can be applied to each segment. The transient DC potentials are superimposed on top of an AC or RF voltage (which acts to confine ions radially) and/or any constant DC offset voltage. The transient DC potentials generate a travelling wave which moves along the axial direction and translates ions along the ion mobility separator.
A known ion mobility separation device comprises a drift tube comprising a series of rings wherein a constant potential difference is maintained between adjacent members such that a constant electric field is produced. A pulse of ions is introduced into the drift tube which contains a buffer gas and ions separate along the longitudinal axis according to their ion mobility. These devices are operable at atmospheric pressure without RF confinement and can offer resolutions up to 150 (Wu et. A. Anal. Chem. 1988, 70, 4929-4938). Operation at lower pressures more suitable for hybrid ion mobility-mass spectrometer instruments leads to greater diffusion losses and lower resolution. An RF pseudo-potential well may be arranged to confine ions radially and may be used to transport ions efficiently by acting as an ion guide and so solving the problem of diffusion losses. Ions may be propelled along the guide and ions may be separated according to their ion mobility. However, the problem of the lower pressure of operation of mobility separation is that in order to achieve a high resolution of mobility separation, a relatively long drift tube must be employed in order to keep within the low field limit as described in more detail below.
In order to separate ions according to their mobility in an RF ion guide a DC electric field must be generated which is orthogonal to the RF radial confinement. If a constant electric field E is applied to drive the ions through the ion guide containing a gas then the ion will acquire a characteristic velocity:vd=E·K  (1)wherein K is the ion mobility.
To achieve a mobility separation whereby the ions acquire negligible energy compared to the background thermal energy of a gas it is necessary to consider the parameter E/P, wherein P is the pressure of the neutral gas.
To maintain mobility separation in the so called low field regime whereby ions do not receive kinetic energy from the driving field it is required that the parameter E/P is less than about 2V/cm-mbar.
Under low field conditions in a drift tube of length L and applied voltage drop V the resolution is found to be independent of ion mobility and only dependent on the voltage drop such that in the absence of space charge effects:
                              L                                                x              _                                                  =                              V                    0.173                                    (        2        )            wherein | x| is the mean displacement of the centre of mass of the moving ion cloud.
The parameter
  L                x      _          is effectively the resolution of the mobility separation so it can be seen that the performance of the spectrometer can be increased by maintaining larger voltage drops across the drift tube. In hybrid ion mobility-mass spectrometer instruments the typical pressure of the ion mobility drift region is 0.5-1 mbar. Operating at pressures much greater than this puts great demands upon the vacuum system which needs to be differentially pumped in order for the mass spectrometer stages to operate efficiently.
At a typical drift tube length of 20 cm and operating pressure of 0.5 mbar; the maximum voltage that can be applied within the low field limit is 20 V giving a maximum resolution of 26. To achieve a resolution of 100 under the same conditions would require a drift tube length of over 3 meters which is impractical for commercial instruments.
It is desired to provide an improved ion guide and ion mobility spectrometer.